Lewis Hamilton spearheaded another one-two for Mercedes in practice, but Daniel Ricciardo is threatening to spoil the German manufacturing giant's party on home soil.

It was a close-run thing between the two championship protagonists as Hamilton finished just 0.024 seconds clear of German team-mate Nico Rosberg following the two 90-minute sessions at a hot Hockenheim ahead of the German Grand Prix.

In scorching temperatures of 32 degrees centigrade, and with the track thermometer hitting a searing 58, H amilton finished with a time of one minute 18.341 seconds after finally getting to grips with the circuit and the loss of FRIC.

Ahead of this race FIA race director Charlie Whiting caused a stir amongst the teams by sending a letter threatening to outlaw the front and rear interconnected (FRIC) device.

FRIC enables the cars to maintain a constant ride height, and is believed to have played a role in Mercedes' dominance so far this season.

Following a detailed investigation of most of the systems, Whiting felt FRIC contravenes one of the FIA's catch-all regulations with regard to moveable aerodynamic devices.

In fear of a rival launching a protest, the FIA confirmed all teams had opted to remove the device prior to practice.

On track it was immediately apparent as to its benefits as a number of cars ran wide in the slow corners due to them being forced to run a higher, stiffer front suspension.

Rosberg and Hamilton were high amongst those who suffered difficulties during the first session, in particular the latter.

Hamilton was comfortably fastest in the opening two sectors, but in the third was off the pace, and so had to settle for second best behind Rosberg.

Come FP2, and after getting used to the new feel of the car, the 29-year-old hit back to narrowly pip Rosberg, who is aiming to complete the best week of his life.

Rosberg, who over the past few days has got married, watched Germany win the World Cup and signed a new long-term deal with Mercedes, is determined to land victory in front of his fans.

Ricciardo, however, may yet throw a spanner in the works as the Red Bull driver finished just a tenth of a second adrift, as close as anyone has got to Mercedes at the end of Friday practice.

Ferrari's Kimi Raikkonen, fit again after damaging ribs and sustaining bruising following his heavy-impact shunt into a barrier on the first lap of the British Grand Prix a fortnight ago, was a distant fourth, half a second down.

Kevin Magnussen - driving a McLaren sporting a new rear wing - and Felipe Massa for Williams were both over 0.6secs adrift, followed by Jenson Button in his McLaren, with the Briton 0.880secs off the pace.

Button was in good company as he was followed by two other world champions in Sebastian Vettel in his Red Bull and Ferrari's Fernando Alonso.

After a successful practice outing for Williams development driver Susie Wolff in FP1, Valtteri Bottas returned to the car for FP2 and was a second adrift in 10th.

Wolff, meanwhile, conjured a strong performance in FP1, finishing only a quarter of a second behind Massa, although was handed two fines for pitlane speeding.

There was also an electrical glitch on her opening installation lap that threatened to scupper the 31-year-old Scot's outing, which would have been an injustice given an engine issue ruined her maiden run at Silverstone after just four laps.

Marussia's Max Chilton finished 3.5secs behind Hamilton in 21st, with Kamui Kobayashi propping up the timesheet after his Caterham caught fire out on track.